Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K3/00—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein
  • F01K3/18—Plants characterised by the use of steam or heat accumulators, or intermediate steam heaters, therein having heaters
• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
  • C10G2/00—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon
  • C10G2/30—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen
  • C10G2/32—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts
  • C10G2/34—Apparatus, reactors
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K13/00—General layout or general methods of operation of complete plants
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K25/00—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for
  • F01K25/08—Plants or engines characterised by use of special working fluids, not otherwise provided for; Plants operating in closed cycles and not otherwise provided for using special vapours
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01K—STEAM ENGINE PLANTS; STEAM ACCUMULATORS; ENGINE PLANTS NOT OTHERWISE PROVIDED FOR; ENGINES USING SPECIAL WORKING FLUIDS OR CYCLES
  • F01K7/00—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating
  • F01K7/02—Steam engine plants characterised by the use of specific types of engine; Plants or engines characterised by their use of special steam systems, cycles or processes; Control means specially adapted for such systems, cycles or processes; Use of withdrawn or exhaust steam for feed-water heating the engines being of multiple-expansion type
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E50/00—Technologies for the production of fuel of non-fossil origin
  • Y02E50/30—Fuel from waste, e.g. synthetic alcohol or diesel

Definitions

• Fischer Tropsch synthesis is a catalyzed process for converting synthesis gas (syngas) containing hydrogen and carbon monoxide into longer chain hydrocarbon products. Since its invention by Franz Fischer and Hans Tropsch during the 1920s, this process and variations on it have been commonly used in coal to liquids and gas to liquids operations.
• the Fischer-Tropsch condensation process is a reaction which converts carbon monoxide and hydrogen into longer chain, mostly paraffinic, hydrocarbons in the presence of an appropriate catalyst and typically at elevated temperatures (e.g., 100 to 350°C, 125 to 300°C, or preferably 175 to 250°C) and/or elevated pressures (e.g., 5 to 100 bar, or preferably 12 to 50 bar).
• Fischer Tropsch reaction is exothermic, yielding roughly 145 kJ per "CH 2 " formed.
• the Fischer-Tropsch process can be used to prepare a range of hydrocarbon fuels, including LPG, naphtha, kerosene and gas oil fractions.
• Fischer Tropsch product selectivity is temperature and pressure dependent, so it is important for the heat of reaction to be quickly removed in order to achieve the desirable product mix and avoid catalyst damage. Increased temperatures may result in undesirable methane selectivity or carbon deposition. Steynberg, Andre, and Mark Dry. Fischer-Tropsch Technology. Amsterdam:4: Elsevier, 2006. Boiling water is commonly used as a coolant in Fischer Tropsch reactors, and the steam formed therefrom can be used to generate shaft work.
• Fischer-Tropsch Synthesis (FTS) reactors reject hundreds of megawatts at 500 K to 650 K, and lesser amounts are rejected in condensers at lower temperatures.” See U.S. Patent Appl. Pub. No. 20100300093A1 (Ser. No. 12/673,554.) filed 25 Nov. 2007 to Doty, F. D. entitled “High-Temperature Dual-Source Organic Rankine Cycle with Gas Separations,” which is herein incorporated by reference. Accordingly, there is an opportunity to develop improved processes for recovering the waste heat from a Fischer Tropsch process using an organic Rankine cycle.
• a process uses a product stream from a Fischer Tropsch synthesis reactor as the coolant in the same Fischer Tropsch reactor. This stream is then used as the working fluid in an associated organic Rankine cycle. In this manner, the waste heat from the Fischer Tropsch reactor can be efficiently converted into shaft work within the Fischer Tropsch plant. The Fischer Tropsch fluid can then be recycled into the reactor product stream.
• a process for the recovery of waste heat includes introducing a stream of a high-pressure organic liquid coolant into a cold side of a reactor heat exchange system, where the coolant provides cooling, preferably at a near-constant temperature condition, of the reactor by vaporizing the coolant into a coolant vapor stream, removing the coolant vapor steam from the cold side of the reactor heat exchange system, introducing the coolant vapor stream into an organic Rankine cycle to generate work and convert the coolant vapor stream into a high pressure liquid coolant, and complete the loop by re-introducing the high pressure liquid coolant into the cold side of the reactor heat exchange system, wherein the organic liquid coolant is derived from a fraction of a hydrocarbon product formed in the reactor.
• the reactor contains a catalytic Fischer Tropsch reaction process. More preferably, the reactor is a fixed bed Fischer Trospch reactor, a fluid bed Fischer Tropsch reactor or a slurry Fischer Tropsch reactor.
• the organic liquid coolant comprises at least 80% by weight of a C6-C12 hydrocarbon, more preferably at least 90% by weight of a C6-C12 hydrocarbon, and even more preferably at least 95% by weight of a C 6 -Ci 2
• the hydrocarbon is non-aromatic C6.
• the reactor is maintained at a temperature between about 100°C and about 350°C, more preferably between about 220°C and about 250°C.
• a process for the recovery of waste heat includes introducing a stream of an organic liquid coolant into a cold side of the reactor heat exchange system, removing the coolant vapor steam from the cold side of the reactor heat exchange system, introducing the coolant vapor stream into an organic Rankine cycle to recover the heat therefrom and convert the coolant vapor stream into a high pressure liquid coolant, introducing the high pressure liquid coolant into the organic liquid coolant stream, removing a product stream from the reactor, separating the product stream into a plurality of fractions including a coolant fraction, introducing the coolant fraction as a make-up into the organic liquid coolant stream, and removing a purge stream from the organic liquid coolant stream.
• the coolant fraction has a boiling point temperature at the operating pressure of the cold side of the reactor heat exchange system which is about the same as the reactor operating temperature.
• the purge stream is introduced into the product stream removed from the reactor.
• FIG. 1 depicts a representative fixed bed Fischer Tropsch reactor with liquid coolant recirculation.
• Fig. 2 depicts a representative process flow diagram of a Fischer Tropsch reactor as a heat source for an organic Rankine cycle.
• Fig. 3 depicts a representative Temperature - Entropy diagram of a Rankine cycle obtained from the process flow of Fig. 2 using n-hexane as the working fluid.
• Fig. 4 depicts a representative Temperature - Entropy diagram of a Rankine cycle obtained from the process flow of Fig. 2 using n-decane as the working fluid.
• Fig. 5 depicts a chart that shows efficiencies for Rankine cycles using several fluids.
• the process of the present disclosure differs from the prior art in that it uses a Fischer Tropsch fluid from the reactor product stream as the coolant in the same Fischer Tropsch reactor, and as the working fluid for an organic Rankine cycle which uses the Fischer Tropsch reactor as its major source of heat.
• This disclosure focuses on a low temperature Fischer Tropsch reaction (220°C - 250°C) in a fixed bed reactor with cobalt-based catalyst targeting a highly paraffinic product stream.
• the process is also applicable to other Fischer Tropsch reactor configurations at this and other temperature ranges between 100°C and 350°C, which may use other catalysts, such as iron-based catalyst for operation at the higher end of the temperature range.
• a distillate fraction taken from the Fischer Tropsch reactor product stream is used as the working fluid for an organic Rankine cycle which has the Fischer Tropsch reactor as its primary heat source. This working fluid can be recycled into the product stream after being used for the organic Rankine cycle.
• a typical fixed bed reactor 10 for a Fischer Tropsch process is shown.
• Such reactors are known in the art. See e.g., U.S. Patent Publication. No. 2008/01 18018 A1 to Schrauwen.
• Syngas feed 12 composed primarily of CO and H 2 enters the reactor 10 and passes through catalyst packed tubes 14 where the reaction occurs.
• the feed stream is synthesized into a mixture of longer chain hydrocarbons ranging from methane (d) up to C6 4 and higher.
• Products flow out the product outlet 16 of the reactor. The products are separated by distillation downstream (not shown).
• coolant In order to maintain a near constant reactor temperature, coolant is boiled at constant pressure on the shell side 18 of the reactor. The heat transfer coefficient is kept high by maintaining a considerable coolant velocity across the tubes.
• a coolant recirculation loop 20 with a recirculation pump 22 maintains high coolant velocity across the tubes even if the overall mass flow of coolant through the reactor is small.
• the liquid coolant inlet stream 24 is introduced into the coolant recirculation stream 20.
• the recirculating coolant stream is maintained at a
• the reactor temperature of between 220°C and 250°C, and more preferably between 230°C and 240°C Over long term operation, the reactor temperature may increase slightly over the life of the catalyst to compensate for catalyst activity loss due to deactivation.
• the pressure on the cold side of the exchanger may need to be adjusted to maintain the same temperature difference between the cold side and hot side in the reactor to balance the heat transfer and coolant flow rates.
• Fresh liquid coolant 24 is added to the coolant loop to balance the coolant that is removed as a saturated vapor 26.
• a representative process flow diagram is disclosed that incorporates a fixed bed Fischer Tropsch reactor 30 as the heat source integrated with equipment for an organic Rankine cycle.
• saturated coolant vapor 32 is drawn off the top of the reactor 30 and used as the working fluid for an organic Rankine cycle.
• the relatively high pressure saturated vapor 32 is passed through an expander 34 to generate shaft work.
• Many organic fluids have a drying nature such that there is no condensation as the fluid is expanded isentropically from a saturated vapor state.
• the expander exit stream 36 is superheated vapor, which can be passed through the hot-side of a recuperator 38 to recover heat from the superheated vapor.
• the lower pressure vapor 40 exits the recuperator 38 and enters a condenser 42, where the vapor is condensed to a saturated liquid stream 44.
• a working fluid pump 46 pumps the condensed liquid stream 44 to become high pressure stream of liquid coolant 48.
• the high pressure liquid coolant stream 48 is preheated in the recuperator 38 before being returned as fresh liquid coolant inlet stream 50 to the reactor coolant recirculation loop 52.
• the mass flow rate of the reactor coolant is the mass flow rate of the reactor coolant
• the coolant recirculation loop 52 is significantly greater than that of the coolant inlet stream 50. This causes the heat capacity rate of the coolant recirculation loop stream 52 to be greater than the heat capacity rate of the liquid coolant inlet stream 50 so that the reactor tubes will not be exposed to significantly colder temperatures.
• the coolant recirculation stream 52 may be in the range of 2 to 20, preferably 2 to 10, times the mass flow rate of the liquid coolant inlet stream 50, depending on reactor and Organic Rankine Cycle ("ORC”) configuration.
• ORC Organic Rankine Cycle
• the "coolant inlet 50" may be at a temperature of about 90°C.
• this temperature may require a very high mass flow rate ratio of coolant recirculation loop to coolant inlet stream to minimize the decrease in the coolant temperature entering the cold side of the reactor exchanger. Therefore, it is desirable to use an embodiment that integrates additional heat sources from the Fischer Tropsch plant, in addition to what is recovered in the recuperator, to raise the coolant inlet stream temperature before it is introduced into the recirculation loop.
• the coolant inlet stream is heated with additional heat sources to about 145°C.
• the coolant inlet stream is introduced and mixes with the recirculation loop, which is saturated liquid at a temperature of 230°C, before the mixture enters the cold side of the reactor exchanger.
• the resulting coolant mixture is no more than 5°C colder than the rest of the coolant in the reactor at 230°C.
• the recirculation loop should have a mass flow rate about 9 times that of the coolant inlet stream.
• the coolant inlet stream may be at a temperature of 188°C.
• the coolant inlet stream mixes with the coolant recirculation loop, which is saturated liquid at a temperature of 230°C. It may be desired in this case that the resulting coolant mixture be no more than 10°C colder than the rest of the coolant in the reactor, at 220°C. To achieve this 220°C temperature, the recirculation loop must have a mass flow rate about 2 times that of the coolant inlet.
• the Fischer Tropsch reactor may supply about 75% of the total heat load for the ORC.
• the remaining 25% heat load may be provided from related processes in other process units associated with the Fischer Tropsch plant.
• Other heat sources at or near the Fischer Tropsch reactor temperature may be used for one or more of the reheat stages between the expander stages.
• Other heat sources at lower temperatures may be integrated into the ORC to provide heat for lower temperature streams, such as the high pressure coolant pumped back to the reactor exchanger.
• a distillation fraction of the product from the Fischer Tropsch reactor is used as the liquid coolant.
• the syngas used as the reactor feed stream 28 may be primarily H 2 and CO, and may have the following composition by mol fraction: 0.308 CO; 0.028 C0 2 ; 0.619 H 2 ; 0.001 H 2 0; 0.038 N 2 ; 0.007 Ar; and trace amounts of 0 2 , CH 4 .
• a typical reaction product formed in the Fischer Tropsch reactor may be mostly hydrocarbons, predominantly n-alkanes, with some isoparaffins and olefins (mostly alpha-olefins) formed but trace, if any, aromatics.
• a typical product stream 54 may have the following composition by mol fraction: 0.178 CO; 0.049 C0 2 ; 0.333 H 2 ; 0.324 H 2 0; 0.063 N 2 ; 0.01 1 Ar; and hydrocarbons, 0.025 (C C 4 ); 0.008 (C 5 -Cn), 0.005 (C 12 - C 22 ), 0.004 (C 2 3-C6 4 ).
• the product stream 54 is sent to a distillation unit 56.
• distillation fractionation units as known in the art for distillation of Fischer Tropsch reaction products may be used. See, e.g. U.S. Patent Nos. 5,486,542 to Posthuma et al. and 6,855,248 to Olivier et al., which are herein incorporated by reference.
• To obtain the distillation cut purity desirable for the organic Rankine cycle of the present disclosure may require subjecting the initial distillation fraction to subsequent distillations or taking a side cut from the distillation tower.
• the desired distillate from the product is sent as fresh working fluid make-up 58 to the organic Rankine cycle portion of the process.
• the fresh working fluid make-up is introduced to the condensed liquid stream 44 at the inlet to the working fluid pump 46.
• a stream of working fluid bleed may be removed from the high pressure working fluid stream 48 at the outlet of the working fluid pump 46.
• the fraction used as make-up may be primarily C6-C12 non-aromatic hydrocarbons, predominantly C6-C12 n-alkanes, or naphtha.
• the fraction may have a boiling point range from 20°C to 220°C, more preferably a boiling point range from 30°C to 100°C.
• the distillate fraction used as make-up may be a full range naphtha fraction having a boiling point range from 20°C to 220°C, or a light naphtha narrower boiling range from 30°C to 100°C.
• the fraction used as fresh working fluid make-up is at least 80% by weight of a C6-C12 hydrocarbon, the same hydrocarbon as used in the working fluid. More preferably, the fresh working fluid make-up is at least 90%, and most preferably at least 95% by weight of a C 6 -Ci 2 hydrocarbon.
• the hydrocarbon is non-aromatic C6 hydrocarbon, such as but not limited to a C6 alkane.
• a representative energy cycle for the organic Rankine cycle corresponding to the process flow diagram of Fig. 2 is depicted.
• the energy cycle 100 is depicted based on a model of the organic Rankine cycle using pure n- hexane as the liquid coolant/working fluid.
• the loci of saturated states for n-hexane are depicted as a dashed line 60 forming a loop on the diagram, where liquid phase 62 is to the left of the loop, vapor phase 64 is to the right of the loop and a mix of saturated liquid and saturated vapor 66 is inside the loop.
• saturated coolant vapor 68 is withdrawn from the reactor.
• the temperature cools under near isentropic conditions to become a superheated vapor 70.
• the superheated vapor passes through a recuperator to further cool down to become a near saturated vapor stream 72 slightly above the saturation temperature.
• the vapor stream 72 passes through a condenser to become a saturated liquid 74.
• the saturated liquid is pumped to a higher pressure 76 and pre-heated in the recuperator to form the liquid coolant stream 78, where it is introduced to the recirculating reactor coolant stream 80.
• the shaft work recovered in this energy cycle is represented by the near isentropic expansion of the saturated vapor from point 68 to 70 on the energy cycle 100.
• a plant process simulation model has been created to study strategies for maximizing resource efficiency by simultaneously producing liquid fuels, chemicals, and power from a coal gasification process.
• Typical plant heat flows indicate an abundance of heat available at low temperatures (less than 250°C) where steam Rankine cycles are known to be inefficient.
• the single largest source of low temperature heat used in the plant process simulation model is the Fischer Tropsch reactor, which converts syngas into useful liquids and waxes. This exothermic reaction rejects a significant heat load, but must be held at a preferred operating temperature of 240°C to produce the desired products.
• the use of organic working fluids in a Rankine cycle is a more efficient option than steam for converting the heat from the Fischer Tropsch reactor into shaft work.
• thermodynamic database compiled by the National Institute of Standards and Technology (NIST) in Refprop 8.0 (Lemmon, Huber, & McLinden, NIST Standard Reference Database 23: Reference Fluid Thermodynamic and Transport Properties-REFPROP, Version 8.0. Gaithersburg, MD, 2007). Fluid states for process side heat loads from the Fischer Tropsch reactor and related processes are calculated using the Peng-Robinson equation of state with the Boston-Mathias alpha function (PR-BM) in Aspen Plus® modeling software by Aspentech.
• PR-BM Boston-Mathias alpha function
• the organic Rankine cycle (ORC) model employed in this discussion for dry fluids operates between a heat source at high temperature, T source , and a heat sink at low temperature, T s ⁇ nk- Fig. 4 shows the fluid states for an organic Rankine cycle on a Temperature vs. Specific Entropy (T-s) diagram. Decane is used as the working fluid in this example. The fluid saturated states which are the boundary of the two-phase region are represented by a dashed line on the T-s diagram. The cycle begins at State 1 as a saturated liquid at low pressure, P tow , and at the cycle minimum temperature, T min . The fluid is pumped up to high pressure, ⁇ , at State 2.
• the high pressure liquid is then preheated from State 2 to State 4, and boiled from State 4 to State 5 by heat transfer from the heat source. (State 3 has been skipped here for reasons that will become clear shortly.) Due to its drying nature, the fluid can be expanded directly from a saturated vapor at State 5 to a superheated vapor state at low pressure at State 6. The low pressure vapor is then cooled and condensed back into a low pressure liquid at State 1.
• the expander exit stream, State 6 in Fig. 4 is a superheated vapor at elevated temperature such that 7 ⁇ 6 > Tmin- Therefore, there is an opportunity to transfer heat from the low pressure vapor stream into the high pressure liquid stream as part of the preheating process that occurs from State 2 to State 4.
• a recuperator is added to recover heat from the superheated expander exit stream, thereby reducing the necessary heat input to operate the cycle.
• the hot vapor enters the recuperator directly out of the expander at State 6, and exits the recuperator at low temperature at State 7.
• the cold stream enters the recuperator as high pressure liquid at State 2 and exits the recuperator at some elevated temperature at State 3.
• Cycle states are calculated as described here.
• State 4 is a saturated liquid at high pressure
• P 4 P h ig h -
• State 2 is defined by first assuming the pump is reversible, yielding state 2 rev which
• the organic Rankine cycle with a dry working fluid has State 3 and State 7 associated with the recuperator.
• the enthalpy of State 3 is determined by evaluating the First Law of Thermodynamics on a control volume surrounding the recuperator.
• Table 2 contains the state information for an organic Rankine cycle as described above.
• the working fluid for this cycle is decane.
• the heat source is waste heat from a plant process at a constant temperature of 240°C
• the heat sink is cooling water at a constant temperature of 30°C
• n 10°C is maintained between the working fluid and the source or sink for all states.
• Table 2 Organic Rankine cycle states for a decane cycle with heat source and sink of 240°C and 30°C respectively.
• the expander and pump have isentropic efficiencies of 0.9.
• Cycle characteristics for this organic Rankine cycle using decane are included in Table 3. Comparative numbers using pure n-hexane and a 90% hexane mixture are also provided. The steam Rankine cycle characteristics are also included to facilitate a comparison between the two cycles.
• Table 3 Organic Rankine cycle characteristics for a decane cycle with heat source and sink of 240°C and 30°C respectively. Steam Rankine cycle results for a comparable cycle are included. Expander and pump efficiencies are assumed to be 0.9.
• Table 3 shows that the organic Rankine cycle achieves superior efficiency to the steam Rankine cycle.
• the specific heat transfer into the cycle, q in , and the specific work output from the cycle, w net are each much greater for water than for decane. This difference is caused by the substantially larger latent heat of water, which is about 20 times that of decane.
• the decane cycle will require a much larger mass flow than the steam cycle to absorb heat at the same rate.
• the pump work input for the organic cycle consumes 3.3% of expander work, whereas the pump work input for the steam cycle consumes less than 0.1 % of expander output. This means that the decane Rankine cycle is more susceptible to pump inefficiencies than the steam Rankine cycle.
• Table 4 contains the state information for an organic Rankine cycle as described above with a working fluid for this cycle comprising a mixture of 90% n- hexane, 5% pentane and 5% heptane.
• the heat source is waste heat from a plant process at a constant temperature of 240°C
• the heat sink is cooling water at a constant temperature of 30°C
• a minimum temperature potential of AT min 10°C is maintained between the working fluid and the source or sink for all states.
• the states correspond to the cycle similar to the decane cycle depicted at Fig. 4.
• Table 4 Organic Rankine cycle states for a 90% n-hexane mixture working fluid with heat source and sink of 240°C and 30°C respectively.
• the expander and pump have isentropic efficiencies of 0.9.
• ORC One of the primary considerations affecting fluid selection is efficiency.
• the purpose of the ORC is to convert heat into shaft work. Steam is more benign and may have better availability, reliability, and environmental impact than any organic fluid choice, so it is desirable that any alternative to steam output more work than a steam Rankine cycle.
• the efficiency of a Rankine cycle depends on the working fluid properties and the heat source and heat sink available to the cycle.
• a minimum temperature difference, M m ; n is maintained between the working fluid and the heat source or heat sink to assure heat transfer.
• dry fluids achieve a higher temperature for heat transfer into the system than wet fluids. Dry fluids do not condense when expanded isentropically from a saturated vapor state. Instead, this expansion results in a superheated vapor. In contrast, wet fluids have a negative slope ⁇ dTlds) sat of the saturated vapor line on the T-s diagram. When a wet fluid is expanded from a saturated vapor state, it begins to condense.
• recuperator From State 6 to State 7, and used to preheat the cold liquid exiting the pump from State 2 to State 3.
• the use of a recuperator negates the disadvantage of the elevated expander exit temperature for a dry fluid rejecting heat.
• the recuperator reduces entropy generation during heat rejection from the cycle, it also reduces entropy generation during heat absorption by the cycle.
• the coldest working fluid temperature at which heat is absorbed without the recuperator is T 2 , but with the recuperator included the coldest temperature at which heat is absorbed is raised to T 3 .
• T 2 The coldest working fluid temperature at which heat is absorbed without the recuperator
• T 3 The coldest temperature at which heat is absorbed
• the heat transfer from the source into the wet fluid occurs from State 2 up to State 5 across a large temperature gradient.
• the heat transfer from the heat source into the recuperated dry cycle occurs only from State 3 up to State 5, across a much smaller temperature gradient.
• the recuperator greatly improves the cycle efficiency for a cycle operating between a constant temperature heat source and a constant temperature heat sink.
• cycle efficiencies are shown for a variety of working fluids for a heat engine operating between a maximum temperature of 230°C and a minimum temperature of 40 °C.
• the efficiencies range from a low of 20.6% for a water to a high of 29.7% for dodecane.
• the Carnot efficiency for a cycle operating at these temperatures is 37.8%.
• Dry fluids exhibit a clear efficiency advantage over wet fluids in Fig. 5. Dry fluids can be further characterized to narrow the field of potential working fluid choices.
• Fluids with higher critical temperatures will often require greater latent heat input than fluids with lower critical temperature.
• This increased latent heat associated with increasing critical temperature, T cr it allows heat to be absorbed at higher temperatures and may result in higher cycle efficiency.
• Baseline ORC efficiency is shown with the working fluid critical temperature in Table 5 below for select fluids. The efficiencies shown are based on the previously stated assumption of a constant temperature heat source at 240°C , a heat sink at 30°C , pump and expander efficiency at 0.9 and the recuperator is included.
• Table 5 Critical pressures, critical temperatures, triple point temperatures and normal boiling point temperatures for selected fluids.
• Any compound chosen as a working fluid must be chemically stable within the temperature and pressure ranges of the cycle. Degradation of a working fluid can change its thermodynamic properties rendering the cycle unpredictable and ineffective. Some products of organic decomposition may cause coking and block equipment passages if not removed. Dehydrogenation can be a particularly challenging issue for hydrocarbons as temperatures increase. Hydrogen gas may become trapped in the condenser, where it interferes with heat transfer and takes up volume. Consequently, the pressure and the temperature in the condenser increase, resulting in a decreased pressure ratio across the expander and a degradation of work output.
• Thermal stability of organic compounds may be greatly reduced by the presence of oxygen, such that air and water leakages into an ORC are undesirable. If the cycle is maintained at a pressure greater than or equal to its environment, all system leakages will be outward.
• the normal boiling point temperature, T bo u helps to quickly identify fluids which will maintain pressures higher than atmospheric pressure as they condense. Select normal boiling temperatures are listed in Table 5. The normal boiling point is the saturation temperature of a fluid at atmospheric pressure, and therefore it is the lowest possible condensation temperature, T min , which can be used if the cycle is to remain above atmospheric pressure.
• any practical system design requires a consideration of the cost for implementation. Fluids should be commercially available, so that the material cost of the working fluid itself is not prohibitive. In an embodiment of the present invention, the working material is prepared from the plant products, so that it need not be commercially available. Also, fluids with low volume flows allow for smaller equipment sizes. Low volume flow fluids can be identified by their high critical temperature, their high critical pressure, and their low boiling temperature.
• a higher critical temperature corresponds to a larger latent heat.
• a fluid with a high critical temperatures will typically require a smaller mass flow than a fluid with a lower critical temperature to achieve a given heat transfer rate into the fluid.
• a high critical pressure, P crit for a fluid corresponds to a high saturation pressure at the maximum temperature. Vapors at high pressure are more dense than vapors at low pressure, so the same mass flow requires a lower volume flow rate than for a fluid at high pressure. If the pressure is too high, then wall thicknesses required for tubes and equipment will begin to drive costs up.
• the critical pressure of water 22064 kPa, provides a reasonable ceiling for working fluid critical pressure, and the organics considered here all have lower critical pressures than water. Select critical pressures are listed in Table 5.
• a low boiling temperature corresponds to a high pressure in the condenser, which increases the density of the fluid in the condenser and decreases the volume flow.
• Table 6 above contains critical pressures for some organics as well as water.
• Desirable organic working fluids may have a T oii between about 20°C and 220°C, preferably between about 30°C and about 100°C.
• Other desirable organic working fluids may have a T cr j t between about 190°C and 350°C.
• the T cr j t should be above the temperature of the vaporized coolant exiting the Fischer Tropsch reactor exchanger, desirably more than 5°C above and preferably more than 10°C above the vaporized coolant temperature of the reactor.
• Mixtures of hydrocarbons fluids meeting these criteria may also be selected as the organic working fluid, though it is preferable to have one predominant component and a narrow T oii range or a narrow T cr jt range for the mixture within the ranges noted above.
• Waste heat from Fischer Tropsch reactors is often used for steam generation.
• medium pressure steam requires superheat which may be unavailable, and low pressure steam is an inefficient medium for converting heat to work.
• An organic Rankine cycle in the configuration described above using hexane as a working fluid has a conversion efficiency of 26.2%. This compares favorably with a steam
• Rankine cycle using the same heat source which has a conversion efficiency of 20.7%.
• Using an organic coolant that is taken from the reactor product stream also has the advantage that leaks between the shell side and the tube side of the reactor have little consequence.
• the Fischer Tropsch plant provides an immediate source of working fluid such that the organic fluid can be continuously recycled to maintain the integrity of the working fluid which might otherwise break down over time.
• the ORC can be further integrated into a Fischer Tropsch plant by constructing a customized cycle that targets the temperature profile available from the FT reactor heat source and other heat sources in the plant.
• a typical Fischer Tropsch plant may include the Fischer Tropsch reactor circuit, the reformer and related processes and equipment, such as product distillation unit and hydrocracker.
• the reactor is the largest single heat source being used, and the other heat sources can range from the temperature at which the ORC condenses (about 20°C minimum) up to roughly 350°C or 400°C at the hottest. At temperatures above 400°C, the organic working fluid will be unstable, and steam will probably start to be more efficient.
• the heat requirement profile of the ORC may be manipulated to match the heat source by routing the working fluid through the cycle to achieve the best possible recuperation. Between multiple expander stages, the working fluid may be reheated within separate passes of the FT reactor exchanger or with other heat sources, to absorb more heat at the FT reactor temperature.
• the ORC may include an extra pressure level to use all of the heat available in the plant to temperatures as low as 100°C.
• the high pressure coolant may be heated up to near reactor temperature with heat sources other than the reactor exchanger, for example the reactor product stream.
• These changes utilize multiple heat sources from the FT plant, where the FT reactor provides about 75% of the ORC heat load. This may yield a customized cycle which converts the FT heat source with an efficiency of 28.5% when using hexane as the working fluid.

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• 2011
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